If you are currently living in the US, you are applying for a spousal visa, NOT FLR(M).
So, to answer your questions about a spousal visa:
1. You will not pay the IHS until you submit the online application... you will be prompted to pay it. The online application is the last thing you do when you are completely ready to mail your documents to Sheffield for processing. You will be given an IHS reference number and you include this with your application.
2 - priority processing is an extra service offered by Sheffield for £551 (currently $771) which gets your application out to the front of the line when it arrives in Sheffield. You purchase it from the VFS Global website before you mail your documents, then you print the receipt, put it on the very top of your documents and write ‘Settlement Priority Service’ in very large letters on the outermost packaging. If you want to purchase your round trip or return shipping labels too (UPS shipping) you can do this on the VFS Global website at the same time ($141 just return or $227 round trip).
3 - you will have 30 days on your visa vignette to enter the UK... the Intended Travel Date you put on the online application will determine when your 30 days starts. You can put a date up to 3 months ahead of the date you submit your online application. If that date passes before your visa is issued, it will be made valid for 30 days from the date it is issued instead.
4 - no, a Marriage Visitor Visa is a 6 month visitor visa that allows you to come to the UK for a visit, get married, and the return to live in the US. You want a Settlement Visa and you select:
Settlement -> Settlement -> Wife (or Husband)... whichever you are
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